---
title: Motivation
---

# Motivation

One of the primary motivations behind Theme UI is to make building themeable,
constraint-based user interfaces in React as simple and as interoperable as
possible. This library builds upon years of work on various libraries, including
[Basscss][], [Tachyons][], [Rebass][], [Styled System][], and various other
internal style guides and design systems. While some of the ideas encapsulated
within this library may seem familiar, the intent here is to combine modern CSS
tooling into a single "mini-framework" with good guidance on how to fall into
the pit of success for creating white-label products, themed component
libraries, and even full websites.

[basscss]: https://github.com/basscss/basscss
[tachyons]: https://tachyons.io
[rebass]: https://rebassjs.org
[styled system]: https://styled-system.com

## MDX

MDX has, in my opinion, quickly become one of the best ways to render Markdown
in React. The ability to render custom React components for any Markdown element
with the `MDXProvider` is a very powerful API and has the potential to open up
how Markdown is leveraged in ways I think we'll continue to see evolve over the
coming years.

While the final rendered HTML _can_ be styled using global CSS or a wrapping
element with child selectors, there are certainly drawbacks to this approach,
and they can lead to unexpected styling bugs when composing themes together.
Using the `MDXProvider` to render custom styled components in MDX is a great way
to avoid this, but the overhead for applying styles in this way can be a lot of
work, even with UI component libraries like Rebass or Material UI. Theme UI
includes the `theme.styles` API as a light abstraction on top of this, that
hopefully feels familiar to people from diverse backgrounds, even those with
little or no experience using MDX.

For examples of previous explorations into this idea, see [Rebass MDX][], [MDX
Style][], and [MDX Blocks][].

[rebass mdx]: https://github.com/rebassjs/mdx
[mdx style]: https://github.com/jxnblk/mdx-style
[mdx blocks]: https://github.com/jxnblk/mdx-blocks

## The `css` and `sx` Props

The `css` prop is a powerful pattern for styling UI in React. It works like the
built-in `style` prop, but it includes some of the best parts of the CSS
language, including media queries and pseudo-classes, and can be authored in
native JavaScript object literal syntax. The `css` prop can be leveraged in a
similar way to the `styled` higher-order component, but also offers more
flexibility when making one-off, context-specific stylistic changes. The `css`
prop also avoids some of the
[pitfalls of mixing CSS properties with HTML attributes](https://github.com/styled-components/styled-components/issues/439).

The `sx` prop is a light abstraction on top of the `css` prop that can serve as
a complete replacement and makes it easier to ensure you use values from your
global `theme` object.

## Why Object Literal Syntax

For many, the choice between using object literal syntax for styles versus
tagged template literals comes down to personal preference, but in the case of
Theme UI, there are some fundamental reasons for using native JavaScript types
for styles.

The `theme` object itself is an object, and keeping styles in a similar format
helps reduce the API surface area. Using and parsing strings that represent
embedded DSLs introduces overhead when mapping over key-value pairs. Theme UI
avoids this overhead for reasons related to performance, testing, and overall
bundle size. For some of the [same reasons](https://facebook.github.io/jsx/)
that React itself uses JSX (i.e. function calls) instead of tagged template
literals, Theme UI only includes support for authoring CSS with object literal
syntax. Additionally, using native JavaScript types has many other benefits that
are outside of the scope of this document.

## Why Emotion

While there are many different solutions to handling CSS in JavaScript, Styled
Components and Emotion have become the most widely-used industry-standard
libraries. If you're building a custom component library, either Styled
Components or Emotion should suit your needs just fine.

For Theme UI, the decision was primarily based on these factors:

- Emotion's implementation of the `css` prop and the custom JSX pragma allows
  for better integration with Theme UI's `sx` prop
- The Emotion API includes more lower-level utilities, like `createEmotion` that
  could be leveraged when considering how multiple themes could be composed
  together
- Emotion's theming context is directly available in `@emotion/react`, allowing
  this library to leverage React's context API in different ways
- In the case of Theme UI internals, the `styled` higher-order component utility
  is not necessarily the best API for creating components, and by not including
  `@emotion/styled` in the core package the bundle size is kept to a minimum –
  i.e. most of the same things can be achieved with the `css` prop

## How is this different from Styled System

Theme UI's `sx` prop was inspired by Styled System, and it uses the same theme
spec that Styled System adheres to. Styled System is a much lower-level API that
is not in any way coupled to React or Emotion. For example, Styled System works
with Node.js, Vue, Svelte, and many other libraries. Theme UI is intended to be
a higher-level abstraction specifically for use in React applications.
